VIII SKULE 209 
various parts of the head. In many Elasmobranchs the roots 
of certain of the anterior spinal nerves perforate the side-walls 
of the occipital region, and indicate the fusion of vertebral 
components with the cranium. In the cranial roof between the 
two periotic capsules there are two small apertures at the bottom 
of a common median depression: through each aperture the 
ductus endolymphaticus (aqueductus vestibuli) passes from the 
vestibular part of the auditory organ to the exterior of the skull. 
Three cartilaginous rods, one from the roof of each olfactory 
capsule, and one, the prenasal or rostral process, from the ethmoid 
cartilage, converge and meet, or nearly meet, in front to form 
the rostrum or support for the preoral or “ cut-water” portion 
of the head. 
The visceral arches are seven in number. The first or man- 
dibular arch consists on each side of an upper portion, the palato- 
pterygo-quadrate or palato-quadrate cartilage, which passes forwards 
in the side-wall of the oral cavity, along the upper margin of the 
mouth, its anterior or palatine part curving inwards to a liga- 
mentous connexion with its fellow beneath the cranial floor. 
Each cartilage has an upwardly directed process (ethmo-palatine 
process) Which is connected bya suspensory e¢hmo-palatine ligament 
with the lateral wall of the cranium behind the lateral ethmoid 
process. The lower or ventral half of the mandibular arch 
(Meckel’s cartilage) is similar in shape to the upper; it articu- 
lates behind with the quadrate portion of the latter by a movable 
joint, and is thence prolonged forwards and downwards in relation 
with the lower margin of the mouth to a median ligamentous 
union with its fellow of the opposite side. The palato-pterygo- 
quadrate and Meckel’s cartilages together form the primitive 
upper and lower jaws, and support the teeth. The hyoid arch 
also consists of a dorsal and a ventral half on each side. The 
dorsal half or hyomandibular element articulates above with the 
periotic capsule. The ventral portion, or cerato-hyal, passes 
downwards and is connected with its fellow by a median copula 
or basi-hyal cartilage situated in the floor of the oral cavity. <A 
series of simple cartilaginous rays (branchial rays) are attached 
to the hinder margins of the hyomandibular and cerato-hyal 
elements. The distal end of the hyomandibular is connected 
by strong ligaments with the hinder portions of both the 
palato-pterygo-quadrate cartilage and Meckel’s cartilage ; in fact, 
VOL. VII P 
